This invention relates to digital optical switches, and in particular is concerned with the improvement of such switches.
Optical space switches play a key role in optical communication networks. They can perform optically transparent network reconfiguration, routing, optical cross-connects and optical packet switching. In combination with passive wavelength multiplexers and demultiplexers, the space switches or switch arrays can be used to realize dynamic reconfigurable wavelength routers and wavelength add-drop multiplexers. As the dense wavelength division multiplexing (DWDM) technology is broadening its applications from mostly long-haul point-to-point transmission systems to metropolitan and local area access networks, compact, low-cost and high-performance optical switches and switch arrays will become more and more important.
Integrated planar waveguide optical switches are very attractive due to their small size, large scalability and potential for monolithic integration with waveguide DWDM (de)multiplexers. Their implementation in InGaAsP/InP material system also allows monolithic integration with semiconductor optical amplifiers that can compensate for the overall loss of the switches. While good performance in terms of loss, crosstalk and speed is important, the integrated waveguide devices have additional challenges in terms of polarization sensitivity, temperature sensitivity and wavelength dependence. A number of waveguide based optical switches have been developed. They include interferometric devices such as directional couplers and Mach-Zehnder interferometers, digital optical switches based on modal evolution in conventional Y-junction branch and based on total internal reflection (TIR), and field-induced waveguide switches.
Directional couplers and Mach-Zehnder interferometers are both based on mode interference effect. They require a precise drive-voltage or current control in order to achieve the switching with a high extinction ratio. The operating voltage or current is dependent on wavelength, thus making multi-wavelengths simultaneous switching impossible in WDM systems. They are also sensitive to polarization and temperature and have small fabrication tolerances.
Digital optical switches (DOS) exhibit a digital transfer response for a wide range of drive voltage/current. To a certain extent, they are independent of wavelength. And they are not sensitive to polarization and temperature. The most commonly used form of digital optical switch is the linear Y-junction branch as illustrated in FIG. 1. Its operating principle is based on adiabatic mode evolution rather than mode interference as in the case of directional couplers and Mach-Zehnder interferometers. In order to ensure adiabatic mode coupling, the angle between the branching waveguides needs to be very small. This leads to a long device length. Since a switch array with a large number of input/output ports requires cascading of many stages of switches, a small device length is highly desirable. The adiabatic coupling also requires that the waveguide structure be weakly guiding so that its index step can be modified by a small index change induced by the drive current/voltage. The cladding layer thickness and etching depth thus needs to be accurately controlled and fabrication-induced strain must be minimized to avoid stress-induced guiding or anti-guiding effects. Another practical issue is the crosstalk. Although several structures have been proposed to improve the crosstalk, no experiment has been reported with crosstalk lower than xe2x88x9220 dB and the theoretically calculated crosstalk is in the order of xe2x88x9225 dB for a single stage switch.
Another form of digital optical switch, based on guided mode total internal reflection (TIR), is illustrated in FIG. 2, which shows the schematic of a TIR switch. Two single mode waveguides intersect at a small angle. A metal contact covering half of the intersection region is deposited on the top surface of the waveguide. When current is injected into the region, the refractive index decreases and a total internal reflection interface is formed which switches the light from one output port to another. The intersection angle is relatively large and therefore compact switch can be fabricated. The switch also exhibits digital response and is insensitive to wavelength and polarization. However, a large refractive index change is required in order to achieve the total internal reflection condition. Furthermore, switching characteristics of the reflection and transmission ports are unsymmetrical, which may limit its use for certain applications.
Another type of switches, based on field-induced waveguides, is schematically shown in FIG. 3. The device consists of an X- or Y-junction. The lateral optical confinement in the input branches is achieved by using ridge waveguides. In the output branches, only metal contacts are formed and no lateral confinement exists initially. When one of the output branches is reverse biased, the refractive index of the region underneath the metal contact increase (through quantum confined Stark effect or carrier depletion effect) and thus the lateral confinement is achieved. The optical signal at the corresponding output port increases due to the waveguiding effect. A major problem with this device is the high loss, since no or little optical power is transferred from the off port to the on port during the switching except for a small coupling effect in the junction region.
Typical X and Y junction switches are described in U.S. Pat. No. 5,148,505 to Hisaharu Yanagawa et al, and an optical switch with curved waveguides is described in U.S. Pat. No. 5,991,475 to William H. Nelson. These switches suffer from the same problems as outlined above. Many other optical switches are described in the prior art, having similar problems. Basically, losses at the switch are now becoming a very important factor in the overall optical system.
The present invention provides an improved optical switch with reduced losses. Broadly a 1xc3x972 switch comprises two intersecting curved output waveguides which connect smoothly with an input waveguide, and a particularly shaped metal contact is positioned on top of each output waveguide, at the intersection. Thus, in accordance with the present invention, an improved optical switch has an input waveguide and two output waveguides diverging from the input waveguide, extending in smooth curves from a junction. One electrode extends across each output waveguides at the junction; the electrodes are separated by a narrow gap. Each electrode on each waveguide extends in a smooth continuation of the inner edge of the other waveguide, to provide a smooth transition from the input waveguide to an output waveguide.